The quality or acceptability of a color copy is a function on how the human eye and mind receives and perceives the colors of the original and compares it to the colors of the copy. The human eye has three color receptors that sense red light, green light, and blue light. These colors are known as the three primary colors of light. These colors can be reproduced by one of two methods, additive color mixing and subtractive color mixing, depending on the way the colored object emits or reflects light.
In the method of additive color mixing, light of the three primary colors is projected onto a white screen and mixed together to create various colors. A well known exemplary device that uses the additive color method is the color television. In the subtractive color method, colors are created from the three colors yellow, magenta and cyan, that are complementary to the three primary colors. The method involves progressively subtracting light from white light. Examples of subtractive color mixing are color photography and color printing. Also, it has been found that electrophotographic printing machines are capable of building up a full subtractive color image from cyan, magenta, yellow and black. They can produce a subtractive color image by one of three methods. One method is to transfer the developed image of each color on an intermediary, such as a belt or drum, then transferring all the images superimposed on each other on a sheet of copy paper. A second method involves developing and transferring an image onto a sheet of copy paper, then superimposing a second and subsequent images onto the same sheet of copy paper. For example, U.S. Pat. No. 4,953,012 discloses an image processing system which can produce a color image by developing the image on a photoconductive surface and transferring an image onto a sheet of copy paper, then superimposing a second and subsequent images onto the same sheet of copy paper.
The third method utilizes the Recharge, Expose, and Develop (REaD) process. In this process, the light reflected from the original is first converted into an electrical signal by a raster input scanner (RIS), subjected to image processing, then reconverted into a light, pixel by pixel, by a raster output scanner (ROS) which exposes the charged photoconductive surface to record a latent image thereon corresponding to the substractive color of one of the colors of the appropriately colored toner particles at a first development station. The photoconductive surface with the developed image thereon is recharged and re-exposed to record the latent image thereon corresponding to the subtractive primary of another color of the original. This latent image is developed with appropriately colored toner. This process (REaD) is repeated until all the different color toner layers are deposited in superimposed registration with one another on the photoconductive surface. The multi-layered toner image is transferred from the photoconductive surface to a sheet of copy paper. Thereafter, the toner image is fused to the sheet of copy paper to form a color copy of the original. For example, U.S. Pat. No. 4,403,848, U.S. Pat. No. 4,599,285, U.S. Pat. No. 4,679,929, U.S. Pat. No. 4,791,455, U.S. Pat. No. 4,809,038, U.S. Pat. No. 4,833,504, U.S. Pat. No. 4,927,724, U.S. Pat. No. 4,941,003, U.S. Pat. No. 4,949,125, U.S. Pat. No. 5,023,632, U.S. Pat. No. 5,066,989 and U.S. Pat. No. 5,079,155 discloses various methods of forming color copies using dry toners, where a first image is formed and developed on a photoconductive surface, the steps above are repeated to superimpose a plurality of toner images on the photoconductive surface, and the toner images is transferred on a copy sheet by one step.
The use of liquid developers in imaging processes is known. For example, U.S. Pat. No. 3,843,538 discloses a developer emulsion comprising a disperse water phase and a continuous phase which is a solution of a pigmented high molecular weight polymer dissolved in an appropriate organic solvent. The emulsion is non-conductive, and may also be stabilized by a surface-active emulsifying agent with a predetermined hydrophilic-lipophilic balance. The liquid component of the emulsion is a solution of polymer resins in an organic solvent of about 90 percent Isopar.RTM. G and 10 percent aromatic hydrocarbons. A release agent, such as polyethylene wax, may be added to assist image transfer. The aqueous component allows for reduction in the amount of isoparaffin solvent which must be evaporated from the photoconductor after transfer. In addition, U.S. Pat. No. 4,659,640, the disclosure of which is totally incorporated herein by reference, discloses a liquid developer containing a volatile liquid carrier, wax, and polyester toner particles. The developer is self-fixing at room temperature as a result of the high wax concentration. Isopar.RTM. G is a preferred liquid carrier, and Epolene is a preferred polyethylene wax.
Liquid developers have many advantages, and often result in images of higher quality than images formed with dry toners. For example, images developed with liquid developers can be made to adhere to paper without a fixing or fusing step, so there is no need to include a resin in the liquid developer for fusing purposes. In addition, the toner particles can be made very small without resulting in problems often associated with small particle powder toners, such as machine dirt which can adversely affect reliability, potential health hazards, limited crushability, and restrictions against the use of coarsely textured papers. Development with liquid developers in full color imaging processes also has many advantages, such as a texturally attractive print because there is substantially no height build-up, whereas full color images developed with dry toners often exhibit height build-up of the image where color areas overlap. In addition, full color imaging with liquid developers is economically attractive, particularly if the liquid vehicle containing the toner particles can be recovered economically and without cross contamination of colorants. Further, full color prints made with liquid developers can be made to a uniformly glossy or a uniformly matte finish, whereas uniformity of finish is difficult to achieve with powder toners because of variations in the toner pile height, the need for thermal fusion, and the like.
When full color images are formed by sequential imaging and development with different colored developers as described in the third method, the ability to maintain consistency of hue in the final image using dry toners depends, in part, in maintaining a substantially constant relationship between exposure and developed mass per area for each color toner layer on photoconductive surface and upon achieving good registration of the several primary color images needed to form the composed color image.
In this process, achieving multiple registering images onto the photoconductive surface with liquid toner is difficult. Generally, liquid images tend to smear and intermingle with one another, distorting and blurring of the full color image. The apparatus and process of the present invention provide a means for forming full color images with excellent registration, thus avoiding the difficulties encountered with many prior art processes.